How to unload a class

I would like to "unload" a class loaded previously by require_once. Mainly I am working on a daemon where handler-classes should be loaded and unloaded on demand. I did not find a way to drop a class from memory and reload it again from a (maybe changed) file. There are some work-a-rounds on the net like using eval or create_function. But is there any clean way to drop a class ? Carsten